Plain English Meaning
The liquid that aircraft hydraulic systems use to push, pull, and hold things in place — like raising the landing gear or applying the brakes. Because liquid does not squash down under pressure, it can carry force from a small pump to a much larger piston that does the actual work.
Full Definition
A specially formulated liquid used in aircraft hydraulic systems to transmit force from one point to another. It is nearly incompressible, lubricates moving parts, resists foaming and oxidation, and is engineered to remain stable across a wide range of temperatures and pressures encountered in flight.
Context Anchor
Seen when learning about aircraft brakes, landing gear systems, leaks, servicing, and .